Electric motor starting circuit



Nov. 17, 1970 E. BOSCH 3,541,364

ELECTRIC MOTOR STARTING CIRCUIT Filed Oct. 4, 1967 ATTORNEY UnitedStates Patent O US. Cl. 31068 7 Claims ABSTRACT OF THE DISCLOSURE Aswitching arrangement for limiting the starting current when operatingportable power tools. A manually actuated switching arrangement isprovided within the portable tool so as to connect a resistor into thecircuit and in series with the power tool, during the transient periodwhen the tool is being started. The switching arrangement is actuatedthrough the manual movement of a lever between two end positions. Thislever located within the gripping area of the handle associated with theportable tool, has also an intermediate position between the two endpositions. When moving the lever to start the power tool, this lever isfirst moved to the intermediate position at which the current limitingresistor is connected into the circuit. This resistor then serves toattenuate the starting peak current. When the lever is moved furthertowards the opposite end position or on position, the resistor isshort-circuited and is effectively out of the power circuit. Theresistor protects the power line from large peak currents which mayresult in the burnout of fuses or circuit breaker drop-out.

BACKGROUND OF THE INVENTION It is well known in the art, that theelectrical machine tools with series wound motors give rise to largestarting currents, when they are designed for higher power requirements.This starting current can, at the instant that the machine tool isswitched on, cause the fuse of the power feed line to burn out. Thissituation may prevail even though the power line and associated fuse maybe more than adequate for normal operation of the electrical machinetool or power tool.

Accordingly, it is an object of the present invention to provide anelectrical power tool, in which the undesired effects arising from theburn-out of fuses when starting the power tool, is eliminated.

To achieve the objects of the present invention, a switching arrangementis provided in which a contact closes a circuit when a switching leveris midway between two end positions. In this midway position of theswitching lever, a resistor is connected between the motor and the powerline feeding the motor. The significance of this arrangement resides inthe condition that the resistor which is thus connected into thecircuit, is designed to adequately attenuate the peak of the currentduring the starting or transient phase. It is sufiicient that thisintermediate swiching position in which the resistor is connected intothe circuit, takes place for only a few hundredths of a second. Thistime interval is readily achieved when moving the switch lever from theoff position to the on position. At the same time, the movement of theswitch lever may be designed to have a delay in it so that apredetermined amount of delay is encountered at the intermediate or3,541,364 Patented Nov. 17, 1970 midway switch position. This delay maybe designed in the form of a mechanical resistance constructed as adetent which must be passed by the switch lever in its motion from theon position to the off position.

The switch arrangement, in accordance with the present invention, isdesigned in a particularly advantageous manner, whereby two furtherswitching contacts are located adjacent to the switching contact whichconnects the resistor into the circuit. The separations between thecontacts which are movable and their corresponding contact position, arearranged so that when the switch lever is actuated or moved, the contactconnected to the resistor and one of these further contacts first arriveat their contact resting position. The second further contact, on theother hand, arrives at its resting position, only when the end positionof the switch lever is attained, corresponding to the on position of thepower tool. At this point, in the switching arrangement, the currentlimiting resistor is short circuited. The switching arrangement is ofparticularly simple construction when the movable contacts are securedto a switch rod which is, in turn, movable or operable against theaction of a spring.

SUMMARY OF THE INVENTION A switching arrangement for limiting thestarting current of electric motors. The normal electric circuit pathbetween the power line feeding the motor and the electromagnetic fieldsof the motor, is subjected to a switching phase in which a resistor isconnected between the motor and the power source. The switchingarrangement includes a switch lever having two end positions for turningthe motor on and off. When the switch lever is brought to anintermediate position from the on position, and on its way to the offposition, the current limiting resistor is connected into the circuitfor limiting the current flow to the motor during its start-up phase. Asthe switch lever is moved further to the on" position, the currentlimiting resistor is short circuited and full current flow is permittedto the motor for normal operating conditions. The current limitingresistor is connected in the circuit for only a short interval of timeto limit the current during the transient period in which the motorexperiences a current peak. The arrangement is particularly adaptable toa portable power tool in which the switch lever is connected or locatedin the handle of the tool. The switching arrangement is thus a safetydevice for assuring that fuses will not burn out when the power tool isconnected to the line and started.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and'its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view of aportable power tool shown in the form of an angular grinder;

FIG. 2 is an electrical schematic diagram of the switching arrangement,in accordance with the present invention, in which the switchingcontacts within the circuit are in their off position;

FIG. 3 is an electrical schematic diagram of the switching arrangementof FIG. 2, and shows the position of the switching arrangement at anintermediate stage in which 3 the current to the motor is limited duringa start-up phase;

FIG. 4 is an electrical schematic diagram of the switching arrangementof FIG. 2, in which the latter is in the on position and wherein fullpower is applied to the motor controlled by the switching arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A grinding disc 1 of an angulargrinder 2 is driven by an electrical motor 3, through means ofmechanical linkage (not shown). The driving motor 3 is, in particular, aseries wound motor. The handle 4 of the grinding machine includes aswitch 5 for the purpose of turning the machine on and off. The switch 5is located in the gripping area and is readily accessible. The switch 5is actuated by means of a switching lever 6 which is movable between oneend position 7 and a second end position 8. The end position 7corresponds to the ofi position on the switch when referring to thecircuit arrangement of FIG. 2. As shown therein, a switch rod 9 has afirst contact 10, an auxiliary contact 11 and a second contact 12, allmovable against their corresponding contact positions 13, 14 and 15,respectively. A resistor 16 is connected between the motor 3 and thecontact position 14.

When the switch lever 6 is moved from the end position 7 to the endposition 8, the switch rod moves in a downward position, in FIG. 2, soas to close the circuits. As a result of this motion of the switch rod9, contacts 10, 11 and 12 are brought into contact with the positions13, 14 and 15. The contacts 10, 11 and 12 are moved in this positionagainst the action of the springs 17, 18 and 19. When the switch lever 6arrives at an intermediate position midway between the end positions 7and 8, while being manually moved, the contacts and 11 assume theposition as shown in FIG. 3, wherein they rest against the contactpositions 13 and 14. In this particular position the contact 12 stillremains separated from its contact position 15.

As a result of this preceding switching arrangement, the connection tothe network 20 is established. However, since the resistor 16 isconnected into the circuit, any transient peak currents, during thestarting phase, are attenuated. The network 20 is a power source fromwhich the motor 3 draws its power and energy for performing its intendedpurpose, as, for example, a grinding operation.

At this midway position a further detent (not shown) can be provided soas to present a definite indexed or detented position for the movementof the switch rod 9. The arrangement may be such that this detent givesrise to a certain amount of mechanical resistance which must be overcomebefore moving the switch rod to its other end position 8.

As soon as the switch lever 6 is located at the end position 8, thecircuit is also closed through the movable contact 12. Thus, in thisposition the movable contact 12 rests against the switching contact 15,as shown in FIG. 4. At this point the current no longer flows to themotor via the resistor 16. This is due to the fact that when the switchlever 6 is in the end position 8 and the movable contact 12 restsagainst the contact position 15, the resistor 16 is short circuitedthrough the path 21. Thus, in this switching configuration, the powerflow takes place directly from the power source 20 to the motor 3, byway of the movable contact 12. In this particular circuit position, theresistor 16 is superfluous, since the current flow under normaloperating conditions will not cause the circuit breaker in the powerfeed line 20 to drop out.

The advantages achieved by the present invention resides particularly inthe condition that operating current when starting the electricalmachine tool, do not result in the burn-out of fuses or the dropping-outof circuit breakers. Furthermore, the present invention achieves thisadvantageous condition with the use of only a single switch in a simplecircuit arrangement. The manual operator or user of the portable machinetool cannot do anything wrong when applying the structural arrangementof the present invention. The handle or grip of the machine tool is,furthermore, substantially equivalent to the handle associated in otherconventional apparatus, and is particularly comfortable. The switchingarrangement, in accordance with the present invention, is especiallyuseful and of advantage when large starting currents may arise as, forexample, in portable electrical saws.

It will be understood that each of the elements de scribed above, or twoor more together, may also find a useful application in other types ofstarting circuits in electrical machine tools differing from the typesdescribed above.

While the invention has been illustrated and described as embodied in astarting circuit in electrical machine tools, it is not intended to belimited to the details shown, since various modifications and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. A switching arrangement for controlling the starting current of anelectric motor, comprising, in combination, a switching member movablebetween an initial, an intermediate, and a final position, saidswitching member being constructed so as to stay in said initial andsaid final position, respectively, when moved into one of saidpositions; first and second movable contact means arranged at apredetermined distance on said switching member in direction of themovement thereof; first and second stationary switching means arrangedalong the path of movement of said switching member, the distancebetween said stationary first and second contact means being greaterthan the predetermined distance between said first and second movablecontact means so that upon movement of said switching member from saidinitial into said final position thereof said first movable contactmeans will contact said first stationary switching means before saidsecond movable switching means contacts said second stationary switchingmeans; and resistor means connected in circuit with said firststationary contact means so that the motor connected in circuit withsaid stationary contact means will, during movement of said switchingmeans from said initial to said final position thereof first beenergized through said resistor means while said second contact meansare spaced from each other and then energized without interposition ofsaid resistor means when said second contact means are in contact witheach other.

2. The switching arrangement for controlling the starting current of anelectric motor as defined in claim 1 including mechanical resistancemeans associated with said switching member for increasing the durationof motion of said switching member.

3. The switching arrangement for controlling the starting current of anelectric motor as defined in claim 2 wherein said mechanical resistancemeans is a detent means.

4. The switching arrangement for controlling the starting current of anelectric motor as defined in claim 1 including switch lever means linkedto said switching mem ber for manually operating said switching memberbetween said initial and final positions.

5. The switching arrangement for controlling the starting current of anelectric motor as defined in claim 1 including spring means acting onsaid switching member for providing mechanical resistance when saidswitching member is moved between said initial and final positions.

6. The switching arrangement for controlling the starting current of anelectric motor as defined in claim 5 wherein said switching membercomprises switch rod means; and switching contact means secured to saidrod means, said switch rod means being surrounded by said spring meansand moving said switching contact means between said initial and finalpositions.

7. The switching arrangement for controlling the start- 2,554,506 5/1951 Schwarz. ing current of an electric motor as defined in claim 6 in-2,373,256 4/ 1945 Miller 318-508 XR eluding lever operating means linkedto said switch rod 3,260,873 7/1966 Wilson et a1. 31072 XR means formanually moving said switch rod means be- 2,348,656 5/1944 Scofield etal. 310-68 tween said initial and final positions. 5 2,103,922 12/ 1937Van Guilden 31068 XR References Cited MILTON O. HIRSHFIELD, PrimaryExaminer UNITED STATES PATENTS B. A. REYNOLDS, Assistant Examiner834,899 11/1906 Freeman 310-681 XR 1,827,372 10/1931 Riddell 310-681 XR10 2,737,371 3/1956 Gerry. 31072;318421

